Light Fraction Carbon Research Articles

Soil organic matter fractions under different vegetation types in permafrost regions along the Qinghai-Tibet Highway, north of Kunlun Mountains, China

As a key attribute of soil quality, soil organic matter (SOM) and its different fractions play an important role in regulating soil nutrient cycling and soil properties.This study evaluated the soil carbon (C) and nitrogen (N) concentrations in different SOM fractions (light– and heavy fractions, microbial biomass) under different vegetation types and analyzed their influencing factors in continuous permafrost regions along the Qinghai-Tibet Highway in the North of Kunlun Mountains, China. Soil samples were collected in pits under four vegetation types — Alpine swamp meadow (ASM), Alpine meadow (AM), Alpine steppe (AS) and Alpine desert (AD) — at the depth of 0-50 cm. The vegetation coverage was the highest at ASM and AM, followed by AS and AD. The results indicated that the concentrations of light fraction carbon (LFC) and nitrogen (LFN), and microbial biomass carbon (MBC) and nitrogen (MBN) decreased as follows: ASM > AM > AS > AD, with the relatively stronger decrease of LFC, whereas the heavy fraction carbon (HFC) and nitrogen (HFN) concentrations were lower in AS soils than in the AD soils. The relatively higher proportions of LFC/SOC and MBC/SOC in the 0-10 cm depth under the ASM soils are mainly resulted from its higher substrate input and soil moisture content. Correlation analysis demonstrated that aboveground biomass, soil moisture content, soil organic carbon (SOC) and total nitrogen (TN) positively correlated to LFC, LFN, HFC, HFN, MBC and MBN, while pH negatively correlated to LFC, LFN, HFC, HFN, MBC and MBN. There was no relationship between active layer thickness and SOM fractions, except for the LFC. Results suggested that vegetation cover, soil moisture content, and SOC and TN concentrations were significantly correlated with the amount and availability of SOM fractions, while permafrost had less impact on SOM fractions in permafrost regions of the central Qinghai–Tibet Plateau.

Long-term cropping system impact on quality and productivity of a Dark Brown Chernozem in southern Alberta

Smith, E. G., Janzen, H. H. and Larney, F. J. 2015. Long-term cropping system impact on quality and productivity of a Dark Brown Chernozem in southern Alberta. Can. J. Soil Sci. 95: 177–186. Long-term cropping system studies offer insights into soil management effects on agricultural sustainability. In 1995, a 6-yr bioassay study was superimposed on a long-term crop rotation study established in 1951 at Lethbridge, Alberta, to determine the impact of past cropping systems on soil quality, crop productivity, grain quality, and the relationship of yield productivity to soil quality. All plots from 13 long-term crop rotations were seeded to wheat (Triticum aestivum L.) in a strip plot design [control, nitrogen (N) fertilizer]. Prior to seeding, soils were sampled to determine soil chemical properties. Total wheat production for the last 4 yr of the study was used as the measure of productivity. The 1995 soil analysis indicated crop rotations with less frequent fallow and with N input had higher soil quality, as indicated by soil organic carbon (SOC) and light fraction carbon (LF-C) and N (LF-N). SOC had a positive relationship to total wheat yield, but was largely masked by the application of N in this bioassay study. Frequent fallow in the previous crop rotation lowered productivity. The concentration of LF-C had a negative relationship, whereas LF-N had a positive relationship to total wheat yield, with and without N fertilization in this bioassay study. Grain N concentration was higher with applied N and when the long-term rotation included the addition of N by fertilizer, livestock manure, annual legume green manure or legume hay. This study determined that long-term imposition of management practices have lasting effects on soil quality and crop productivity.

Soil extracellular enzyme activities are sensitive indicators of detrital inputs and carbon availability

In a litter manipulation experiment in a temperate deciduous oak forest in Central Europe, we examined soil carbon contents and density fractions, as well as β-glucosidase and polyphenol oxidase enzyme activities, which play central roles in the degradation of litter and soil organic matter. Our measurements were taken in the DIRT (detritus input and removal treatments) plots, where manipulations include doubling of leaf litter or woody debris inputs, as well as removal of litter, trenching to prevent root inputs, and removal of all litter inputs. After 10 years of manipulation, soil C content did not vary predictably among plots, although the amount of light fraction material was greater in control and litter addition plots compared to litter removal plots. Even after 10 years of litter addition, there were no significant differences in activities of either enzyme in double litter plots compared to control plots, a result consistent with other observed measures of microbial activity. However, removal of roots and litter caused significant decreases in β-glucosidase activities very quickly, and these differences increased over time. However, polyphenol oxidase activities were not significant different among treatments. Enzyme activities were not correlated with total soil carbon contents, but activities of both enzymes were significantly and positively related to the amount of light fraction carbon, suggesting that enzymes respond to increases in labile carbon availability.

Tillage and residue management effects on temporal changes in soil organic carbon and fractions of a silty loam soil in the North China Plain

AbstractSoil degradation and associated depletion of soil organic carbon (SOC) have been major concerns in intensive farming systems because of the subsequent decline in crop yields. We assessed temporal changes in SOC and its fractions under different tillage systems for wheat (Triticum aestivum L.) – maize (Zea mays L.) cropping in the North China Plain. Four tillage systems were established in 2001: plow tillage (PT), rotary tillage (RT), no‐till (NT), and plow tillage with residues removed (PT0). Concentrations of SOC, particulate organic carbon (POC), non‐POC (NPOC), labile organic carbon (LOC), non‐LOC (NLOC), heavy fraction carbon (HFC) and light fraction carbon (LFC) were determined to assess tillage‐induced changes in the top 50 cm. Concentrations of SOC and C fractions declined with soil depth and were significantly affected by tillage over time. The results showed that SOC and its fractions were enhanced under NT and RT from 0 to 10 cm depth compared with values for PT and PT0. Significant decreases were observed below 10 cm depths (P < 0.05) regardless of the tillage system. The SOC concentration under NT for 0–5 cm depth was 18%, 8%, and 10% higher than that under PT0 after 7, 9, and 12 yr of NT adoption, respectively. Apparent stratification of SOC occurred under NT compared with PT and PT0 for depths >10 cm. All parameters were positively correlated (P < 0.01); linear regressions exhibited similar patterns (P < 0.01). Therefore, to maintain and improve SOC levels, residue inputs should be complemented by the adoption of suitable tillage systems.

Fracionamento físico da matéria orgânica e índice de manejo de carbono de um Argissolo submetido a sistemas conservacionistas de manejo

O fracionamento físico da matéria orgânica (MO) do solo é uma alternativa para a obtenção de frações lábeis utilizadas no cálculo do índice de manejo de carbono (IMC). O objetivo deste trabalho foi avaliar a eficiência do fracionamento físico granulométrico (53µm) e densimétrico, com soluções de iodeto de sódio 1,8Mg m-3 (NaI) ou politungstato de sódio 2,0Mg m-3 (PTS), na separação das frações lábeis (particulada e leve, respectivamente) da MO e o seu uso na estimativa do IMC. Amostras da camada de 0-20cm de um Argissolo Vermelho sob preparo convencional (PC) e plantio direto (PD), combinados com dois sistemas de culturas (aveia/milho e aveia+ervilhaca/milho+caupi), foram avaliadas quanto ao estoque de carbono orgânico total (COT), e nas frações particulada (COP) e leve (C-FL) da MO. No fracionamento físico densimétrico, com a utilização de NaI, foi recuperado menos C-FL em relação ao uso de PTS. A recuperação de COP no fracionamento granulométrico foi intermediária entre a recuperação de FL-NaI e de FL-PTS. O IMC com os dados do fracionamento granulométrico apresentou elevada correlação com o IMC dos dados de fracionamento densimétrico (NaI e PTS), havendo porém uma subestimação dos resultados com o uso de NaI. Os sistemas de manejo sem revolvimento do solo e com maior aporte de resíduos vegetais resultaram em maior IMC, tanto pelo aumento do índice de labilidade quanto do índice de estoque de carbono. O IMC a partir de frações físicas densimétricas da MO demonstra ser um índice eficiente na discriminação de sistemas de manejo, podendo ser utilizado na avaliação das práticas de manejo do solo.

Can δ13C abundance, water-soluble carbon, and light fraction carbon be potential indicators of soil organic carbon dynamics in Zoigê wetland?

Zoigê alpine wetland located at the Qinghai–Tibet Plateau, China, is the largest alpine wetland in the world and very sensitive to global climate changes. However, little is known of the dynamics of soil water soluble organic carbon (WSOC), light fraction organic carbon (LFOC), soil organic carbon (SOC) and its indicators in this alpine wetland. In this study, four wetland sites and one degraded wetland site in the Zoigê alpine wetland were selected to investigate the relationships of stable isotope and labile organic carbon dynamics with groundwater elevation or water table level. We also examine if the δ13C abundance, WSOC, and LFOC can be used as potential indicators of SOC dynamics in these wetland sites. The results showed that the plant dominant species in the degraded wetland site were different from those in the non-degraded wetland sites. Moreover, the declining water table did not significantly lower foliar δ13C, C and N concentrations and C/N ratio of dominant plant species, but led to a significant decrease of soil SOC, WSOC and LFOC concentrations and an increase of soil δ13C. In the wetland sites, SOC was significantly correlated with soil δ13C, WSOC, and LFOC. Our regression analysis showed that LFOC was the best potential indicator of SOC dynamics with WSOC and soil δ13C being less useful as indicators, respectively. In addition, the absolute slope coefficients of the regression models increased as the soil depth increased, which indicated that the SOC turnover increased with soil depth.

A dual isotope approach to isolate soil carbon pools of different turnover times

Abstract. Soils are globally significant sources and sinks of atmospheric CO2. Increasing the resolution of soil carbon turnover estimates is important for predicting the response of soil carbon cycling to environmental change. We show that soil carbon turnover times can be more finely resolved using a dual isotope label like the one provided by elevated CO2 experiments that use fossil CO2. We modeled each soil physical fraction as two pools with different turnover times using the atmospheric 14C bomb spike in combination with the label in 14C and 13C provided by an elevated CO2 experiment in a California annual grassland. In sandstone and serpentine soils, the light fraction carbon was 21–54% fast cycling with 2–9 yr turnover, and 36–79% slow cycling with turnover slower than 100 yr. This validates model treatment of the light fraction as active and intermediate cycling carbon. The dense, mineral-associated fraction also had a very dynamic component, consisting of ∼7% fast-cycling carbon and ∼93% very slow cycling carbon. Similarly, half the microbial biomass carbon in the sandstone soil was more than 5 yr old, and 40% of the carbon respired by microbes had been fixed more than 5 yr ago. Resolving each density fraction into two pools revealed that only a small component of total soil carbon is responsible for most CO2 efflux from these soils. In the sandstone soil, 11% of soil carbon contributes more than 90% of the annual CO2 efflux. The fact that soil physical fractions, designed to isolate organic material of roughly homogeneous physico-chemical state, contain material of dramatically different turnover times is consistent with recent observations of rapid isotope incorporation into seemingly stable fractions and with emerging evidence for hot spots or micro-site variation of decomposition within the soil matrix. Predictions of soil carbon storage using a turnover time estimated with the assumption of a single pool per density fraction would greatly overestimate the near-term response to changes in productivity or decomposition rates. Therefore, these results suggest a slower initial change in soil carbon storage due to environmental change than has been assumed by simpler (one-pool) mass balance calculations.

Soil organic matter characteristics, biochemical activity and antioxidant capacity in Mediterranean land use systems

The characteristics of soil quality were measured in adjacent agricultural (horticultural cropping sequence, HC), native grassland (naturally grazed, NG) and forest (indigenous wood of holm-oak, F) soils. The objective of the research was to assess the influence of different land uses on soil organic matter characteristics, biochemical activity and antioxidant capacity in selected fields of the Mediterranean environment in central Italy under a specific climatic regime.Land use induced significant changes in the content and quality of soil organic matter, biochemical activity and antioxidant capacity, with more pronounced differences between soils under HC and F than soils under HC and NG. The HC soil showed the lowest amounts of total organic carbon (TOC), microbial biomass C (MB-C), water-soluble organic C (WSOC), water- and alkali-soluble phenols. The organic matter of HC was characterized by the lowest percentage of MB-C and of light fraction carbon (LF-C). The dehydrogenase activity (DH-ase), metabolic potential (MP), hydrolyzing coefficient (HyC), potentially mineralizable C (C0) and C mineralized (Cm) were clearly lower in HC. The specific respiration activity of biomass (qCO2) was the highest in HC soil (1.3μg CO2–CμgbiomassC−1) and lowest in F soil (0.5μgCO2–CμgbiomassC−1) and was inversely related with pH, TOC and MB-C contents. The antioxidant capacity of soils (TEAC) was the highest in NG and related to the amount of alkali-soluble phenols. The rate constant of organic matter mineralization (k) appeared to depend on TEAC rather than the relative amounts of the labile C pools. These results seem to explain the role of phenols as controller of the mineralization rate of organic matter.

耕作方式对紫色水稻土轻组有机碳的影响

Physical fractionation of soil organic carbon(SOC) has been useful in distinguishing specific carbon pools that are responsive to management and identifying the physical control of organic matter.Density fractionation allows the separation of SOC fractions from different locations in the soil and degree of association with mineral particles.The free light fraction carbon(F-LFC) includes loose organic particles in the soil and particulate organic carbon adhering to the exterior of secondary organomineral complexes,which is chemically very close to litter and has generally a very fast decomposition rate.The occluded light fraction carbon(O-LFC) is the fraction of uncomplexed organic matter that is trapped and physically protected within secondary complexes(intra-aggregate organic carbon).This fraction shows a more advanced degree of decomposition and a slower turnover.The heavy fraction carbon(HFC) is composed mainly of highly decomposed materialand has a very slow decomposition rate.Changes in these different fractions,especially in the low-density fractions,may be more sensitive to management-induced impacts on soil than the total SOC or the complicated heavy fraction organic carbon.However,there are few direct data quantifying these two light fractions and their contributions to total SOC storage as related to changes in tillage practices. The impact of tillage on SOC fractions has been of great interest in recent years.The tillage experiment was located at the farm of Southwest University(30°26′N,106°26′E) in Chongqing since 1990.In this paper four tillage treatments including conventional tillage with rice only system(DP),conventional tillage with rotation of rice and rape system(SH),no-till and ridge culture with rotation of rice and rape system(LM),and ridge culture with rotation of rice and rape system(LF) were selected as research objectives to measure the contents of different soil carbon fractions.Soil sampling at the depths of 0—10,10—20,20—30,30—40,40—50 and 50—60 cm was performed with a soil drill after the rape harvest in the spring of 2009. Results indicated that most carbon(69.6%—92.7%) was held in the HF,carbon dynamics in the whole soil were controlled by the behavior of this fraction.The proportion of F-LF carbon to the total SOC was 5.03%—26.43%.O-LF contained the least amount of organic carbon(1.37%—4.93%) of total SOC.In the four tillage soils the greatest contents of F-LFC,O-LFC and HFC were found in the 0—10cm depth.In the whole soil layer(0—60cm),the order of F-LFC contents was LM(4.36g/kg) DP(2.11g/kg) LF(1.74 g/kg) SH(1.46 g/kg),and the corresponding proportions of F-LFC to total SOC were 17.12,14.00,12.17 and 11.29%,respectively.The order of O-LFC contents was LM(0.82 g/kg) DP(0.51 g/kg) LF(0.36 g/kg) SH(0.34 g/kg),and the corresponding proportions were 3.36,3.45,3.00 and 2.71%,respectively.Compared with the other tillage systems,LM treatment significantly increased the contents and proportions of the two light fraction carbons which suggested LM could improve the soil quality in a purple paddy soil in Southwest China.Additionally,the data analysis showed that F-LFC was the most sensitive pool of organic carbon to detect changes in SOC due to soil tillage.

Ridge-furrow and plastic-mulching tillage enhances maize–soil interactions: Opportunities and challenges in a semiarid agroecosystem

Understanding plant–soil relationships may help maximize crop productivity while maintaining and improving soil quality. Field experiments were conducted in 2006 and 2007 at the Dryland Agricultural Experimental Station of the Loess Plateau, Lanzhou University, China, to determine the effects of various ridge-furrow and plastic-mulching techniques on the growth and yield of maize ( Zea mays L.) and soil biochemical properties. Five treatments were designed: (1) flat-plot sowing without ridge-furrow mulching (CK), (2) large (80 cm) and small (40 cm) ridges alternated and fully mulched with plastic (DRM), (3) on-furrow sowing with plastic mulch applied only on the ridge at a row spacing of 60 cm and 40 cm alternatively (RM), (4) flat-plot sowing with plastic mulch at a row spacing of 60 cm and 40 cm alternatively (NM), and (5) flat-plot sowing with plastic mulch at a row spacing of 80 cm and 40 cm alternatively (WM). The results showed that film mulching enhanced soil microbial biomass; where microbial biomass carbon (MBC) in the DRM treatment reached 633 mg kg −1 at harvest in 2007, three times the MBC of the CK. The MBC:SOC ratios were 8.8%, 7.1%, 5.7% and 5.4% in DRM, RM, NM and WM, respectively. The ridge-furrow with plastic-mulching increased soil light fraction carbon (LFOC) in both years, averaging up to 1.04 g kg −1 at harvest. Underground plant biomass increased substantially in the mulching treatments, especially in DRM. Positive correlations were found between total biomass and LFOC, between MBC and LFOC, and between MBC and available phosphorus (AP), but a negative correlation between SOC and soil mineral nitrogen (MN). The carbon to phosphorus (C/P) ratio was highest in DRM among treatments, but the content of SOC, MN, and C/N ratio in DRM was lowest, suggesting that the DRM treatment strengthened the interactions between maize and soil, and that the increased content of LFOC with time provides a basis for increasing productivity in future years. In conclusion, the ridge-furrow and plastic-mulching technique brought about a challenge in maintaining soil fertility, but this technology provides a potential opportunity of substantially increasing crop yields in semiarid rainfed regions.

Comparison of Soil Organic‐Matter Characteristics under the Energy Crop Giant Reed, Cropping Sequence and Natural Grass

The objective of this study was to determine, on the same pedological, topographic, and climatic conditions, the effects of a continuous giant reed cropping (GR), a cropping sequence (CS), and untilled natural grass (NG) on some soil characteristics. Selected characteristics included total soil organic carbon (TOC), light fraction carbon (LFC), dissolved organic carbon (DOC), and microbial biomass carbon (MBC). The various humic fractions and their ratios, humification ratio (HR), humification degree (HD), and humification index (HI) were also evaluated. Results indicate that GR, characterized by low tillage intensity, showed greater values than tilled management system (CS) for all the previously mentioned properties, with the exception of the humification parameters, which were not statistically different between the two systems. Regarding the difference between GR and NG (the latter usually considered as a reference or baseline for evaluating attributes of soil quality), GR generally showed greater values for all organic parameters, with the exception of MBC and humus characteristics.

Soil organic carbon pools after seven years of manures and mineral fertilizers application in a rice-wheat rotation

A long-term experiment under a rice-wheat system, which represents an alternate flooding (reducing) and upland (oxidation) conditions, was used to evaluate the effect of organic manures and mineral fertilization on various organic carbon pools in soil. Application of mineral fertilizers alone increased the dissolved organic carbon (DOC), microbial biomass carbon (MBC), light fraction carbon (LFC), and heavy fraction carbon (HFC) by a factor of 1.4–2.4 compared with control. On the other hand, application of farmyard manure, press mud and green manure along with mineral fertilizers enhanced DOC content by 73.0, 93.9, and 56.5%, respectively, over mineral fertilizer only (N150P75K75Zn25) treatment. The increments in MBC due to corresponding treatments over mineral fertilizers were 73.7, 79.6 and 30.2%, respectively. A maximum increase of 2.5- to 3.4-fold was observed in LFC due to combined application of organic manures and mineral fertilizers over control. Notably, the HFC constituted the major proportion (about 83.3%) of the total soil carbon. The increase in surface soil carbon accounted for 24.9, 28.3 and 7.0% of the carbon added through farmyard manure, press mud and green manure, respectively.

Labile soil organic matter pools in a calciorthent after 18 years of afforestation by different plantations

Labile soil organic matter pools (LSOMs) are the fine indicators of soil quality which are influenced by changes in management practices. The suitability of forest tree species is essential for soil quality improvement of a nutrient deficient calciorthent with very high percentage of free CaCO 3 (34%). Six multipurpose tree species were selected to investigate the effect of afforestation after 18 years of plantations on size and dynamics of LSOMs, e.g. dissolved organic carbon (DOC), microbial biomass carbon (MBC) and light fraction carbon (LFC) and also on soil respiration. LSOMs were estimated during four seasons: summer (June), autumn (September), winter (December) and spring (March) in 0–15 cm and 15–30 cm soil depths and in situ soil respiration was also measured during the corresponding periods. Afforestation by Eucalyptus tereticornis (Smith) increased soil organic carbon (SOC) of the surface 30 cm soil layer by 142% which was at par with Terminalia arjuna Bedd. and Albizia procera (Roxb.) Benth while 63% increase was found by Pongamia pinnata (Linn.) over control plot (3.10 kg ha −1 SOC). Soil under T. arjuna had the highest increase in the mean annual concentration of DOC (by 201%), MBC (by 413%) and LFC (by 263%) over control in the 30 cm soil layer followed by E. tereticornis. Among the LSOMs, LFC had the maximum contribution to SOC (9.61–11.71%) in the afforested plots up to 30 cm soil layer. The relationships among the three labile pools were highly significant. Mean annual soil respiration was highest in E. tereticornis plot which was at par with Acacia lenticularis (L.) Willd and A. procera, while it was lowest in T. arjuna plot. Thus, MBC was the most sensitive to afforestation and LFC was the largest reservoir of carbon among the three LSOMs. The results suggest that afforestation by the six multipurpose tree species had increased the LSOMs and soil respiration to different magnitudes. T. arjuna was found to be the most suitable plantation with highest increase in LSOMs and lowest respiration among the different plantations.

Impact of long-term additions of chemical fertilizers and farm yard manure on carbon and nitrogen sequestration under rice-cowpea cropping system in semi-arid tropics

Restoration of soil organic carbon (SOC) in arable lands represents potential sink for atmospheric CO2. The strategies for restoration of SOC include the appropriate land use management, cropping sequence, fertilizer and organic manures application. To achieve this goal, the dynamics of SOC and nitrogen (N) in soils needs to be better understood for which the long-term experiments are an important tool. A study was thus conducted to determine SOC and nitrogen dynamics in a long-term experiment in relation to inorganic, integrated and organic fertilizer application in rice-cowpea system on a sandy loam soil (Typic Rhodualf). The fertilizer treatments during rice included (i) 100% N (@ 100 kg N ha−1), (ii) 100% NP (100 kg N and 50 kg P2O5 ha−1), (iii) 100% NPK (100 kg N, 50 kg P2O5 and 50 kg K2O ha−1) as inorganic fertilizers, (iv) 50% NPK + 50% farm yard manure (FYM) (@ 5 t ha−1) and (v) FYM alone @ 10 t ha−1 compared with (vi) control treatment i.e. without any fertilization. The N alone or N and P did not have any significant effect on soil carbon and nitrogen. The light fraction carbon was 53% higher in NPK + FYM plots and 56% higher in FYM plots than in control plots, in comparison to 30% increase with inorganic fertilizers alone. The microbial biomass carbon and water-soluble carbon were relatively higher both in FYM or NPK + FYM plots. The clay fraction had highest concentration of C and N followed by silt, fine sand and coarse sand fractions in both surface (0–15 cm) and subsurface soil layers (15–30 cm). The C:N ratio was lowest in the clay fraction and increased with increase in particle size. The C and N enrichment ratio was highest for the clay fraction followed by silt and both the sand fractions. Relative decrease in enrichment ratio of clay in treatments receiving NPK and or FYM indicates comparatively greater accumulation of C and N in soil fractions other than clay.

Light Fraction Carbon and Water-Stable Aggregates in Black Soils

The distribution of light fraction carbon (LF-C) in the various size classes of aggregates and its relationship to waterstable aggregates as well as the influence of cultivation on the organic components in virgin and cultivated black soils were studied by wet sieving and density separation methods. The total organic carbon (TOC) and LF-C were significantly higher ( P ≤ 0.05) in the virgin soils than in the cultivated soils. The LF-C in aggregates of different size classes varied from 0.9 to 2.5 g kg −1 in the cultivated soils and from 2.5 to 7.1 g kg −1 in the virgin soils, whereas the ratio of LF-C to TOC varied from 1.9% to 7.3% and from 5.0% to 12.2%, respectively. After being incubated under constant temperature and controlled humidity for three months, the contribution of LF-C to TOC sharply decreased to an amount (1.7%–8.5%) close to the level in soils that had been cultivated for 20 to 25 years (1.3%–8.8%). As a result, the larger water-stable macro-aggregates (especially > 1 mm) decreased sharply, indicating that the LF-C pool in virgin soils declined quickly after cultivation, which reduced the water stability of soil aggregates.

Atrazine degradation in soils: the role of microbial communities, atrazine application history, and soil carbon

The degradation rate of atrazine in floodplain soils under natural grasslands and cropped fields in the Liverpool Plains, NSW, was studied under laboratory incubation and in glasshouse bioassays, and related to soil properties including microbial community analysis by t-RFLP. The experiments were part of a broader study aiming to manage pesticides in the environment using vegetative filters (biofilters). The soils differed in their atrazine treatment history. Degradation rate (half-life) in cropped soil was more rapid (≈2 to 7 days) than in 2 grassland soils (≈8 to ≈22 days). Bioassays in the glasshouse using oats and soybeans supported this finding. The t-RFLP analysis disclosed the presence of 2 consortia of bacterial species that are reported in the literature to degrade atrazine. These were: (i) Rhodococcus sp, Pseudomonas aeruginosa, and Clavibacter michiganense and (ii) Acinetobacter sp., Pseudomonas sp., and Streptomyces sp. Their dynamics during incubation suggested that they might have been responsible for the more rapid atrazine degradation in the cropped soil. The enhanced atrazine degradation in cropped soil was also associated with lower concentrations of soil organic C and percentage of light fraction carbon compared with grassland soils, suggesting that atrazine provided an additional source of N and C to organisms that can quickly degrade the herbicide. The finding of relatively short atrazine half-life has implications for the effectiveness of the herbicide, as well as for the loads of pesticide potentially entering the environment. The results suggest there is little risk of atrazine accumulating in biofilters and causing damage.

Comparison of Labile Soil Organic Matter Fractionation Techniques

Labile soil organic carbon (SOCL), soil organic carbon with a relatively short turnover time, is an important source of energy for the belowground portion of ecosystems and is sensitive to land management changes. Many techniques exist to differentiate and quantify labile SOC, but rarely have these been directly compared. Here we compare the results of four common chemical, physical, and biological methods of empirically measuring labile SOC with soils taken from 33 restored grasslands that differ in length of time since cessation of agriculture. Among sites, microbial biomass C, acid‐hydrolyzable C, the amount of C respired after 12 d of a laboratory incubation, and light fraction carbon (LFC) were all positively correlated with one another, although there were large differences in the sizes of the pools estimated with each method. Acid‐hydrolyzable C consistently provided the largest estimate and 12‐d incubations the smallest estimate of labile SOC. The quantity of labile SOC obtained by fitting respiration data from a laboratory incubation with a two‐pool model with separate decay constants for each pool was also positively correlated with the three measures of labile soil C not derived from respiration data, although this technique was sensitive to whether the decay constant of the recalcitrant pool was constrained or not. All methods showed increases in labile SOC pools with increases in total soil organic carbon (SOCT) pools, although the rate of change varied between techniques. The size of stable aggregates correlated positively with hydrolyzable C and SOCT, supporting the idea that aggregates may physically protect soil C from decomposition, although the degree to which this C is labile is unclear.

Estoques totais de carbono orgânico e seus compartimentos em argissolo sob floresta e sob milho cultivado com adubação mineral e orgânica

Os estoques de matéria orgânica do solo e seus compartimentos são importantes na disponibilidade de nutrientes, agregação do solo e no fluxo de gases de efeito estufa entre a superfície terrestre e a atmosfera. Os objetivos deste estudo foram: (a) avaliar os efeitos de sistemas de produção de milho sob adubação orgânica e mineral nos estoques totais de carbono orgânico (COT) e nitrogênio (NT) e de compartimentos de carbono (C) orgânico, em um Argissolo Vermelho-Amarelo, e (b) estimar a contribuição desses sistemas no seqüestro ou emissão de CO2 atmosférico. Os sistemas de produção, durante 16 anos, constaram de combinações entre dois níveis (0 e 1) de composto orgânico, nas doses de 0 e 40 m³ ha-1 (AO), e três níveis (0, 1 e 2) de adubo mineral, nas doses de 0, 250 (AM1), e 500 kg ha-1 (AM2) da fórmula 4-14-8. Uma área sob Floresta Atlântica (FA) adjacente ao experimento foi amostrada e usada como referência de um estado de equilíbrio. Os sistemas de produção em que o composto orgânico foi adicionado apresentaram maiores estoques de COT, NT, carbono da fração leve (C FL) e carbono lábil (C L) do que os sistemas sem adubação ou apenas com adubação mineral, o que confirma a adubação orgânica como estratégia de manejo importante para a melhoria da qualidade do solo. No entanto, no solo sob FA, os estoques de COT, NT e dos compartimentos de C foram maiores do que aqueles observados nos sistemas de produção. Em virtude da maior sensibilidade, os estoques dos compartimentos do C FL e do C L foram reduzidos em maior intensidade do que os estoques de COT, razão por que podem ser usados como indicadores da interferência antrópica ou das mudanças no manejo sobre o estado da matéria orgânica do solo.

Carbon storage and light fraction C in a grassland Dark Gray Chernozem soil as influenced by N and S fertilization

A field experiment was conducted at Canwood, Saskatchewan to determine if annual applications of nitrogen (112 kg N ha−1) and sulphur (11 kg S ha−1) fertilizers to grass over 13 yr (1980 to 1992) could increase storage of total C (TC) and light fraction C (LFC) in a Dark Gray Chernozem soil. Hay was removed from the plots every year. The increase in mass of TC in the 0- to 30-cm soil depth from NS fertilizer was 3.88 Mg C ha−1. However, the increase in mass of LFC from NS was 9.50 Mg C ha−1, and most of the increase was found in the 0- to 5-cm soil depth. Key words: Soil carbon storage, light fraction carbon, N and S fertilizers

Crop‐Pasture Rotation for Sustaining the Quality and Productivity of a Typic Argiudoll

AbstractInclusion of pastures in a rotation may reverse the effects of conventional cropping and tillage on soil degradation. We hypothesized that crop‐pasture rotations could be defined with a minimum pasture component and a maximum conventional cropping component to accomplish sustainable management. Soil organic carbon (SOC), light‐fraction carbon (LFC), microbial biomass nitrogen (MBN), and aggregate stability index (ASI) were measured in the surface 0 to 0.15 m of a fine, mixed, thermic Typic Argiudoll with 2% slope under a long‐term crop rotation experiment at Balcarce, Argentina. Treatments were continuous cropping and crop‐pasture (50:50 and 75:25) rotations. All seedbeds were prepared with conventional tillage. All soil quality indicators decreased with cropping and increased with pasture. Data for each variable were fitted to an exponential model to describe their variation in time. Soil organic C decreased 4.4 g kg−1 in 6 to 7 yr under cropping and rose to the original level (37.2 g kg−1) after 3 to 4 yr under pasture. Light‐fraction C, MBN, and ASI fell 0.9 g kg−1, 39.0 mg kg−1, and 43.9, respectively (97–100% of the estimated decline), after 7 yr under cropping, while they recovered to the values at the beginning of the cropping period (1.8 g kg−1, 99.5 mg kg−1, and 76.7, respectively) in a few years under pasture. Rotations including a maximum of 7 yr of conventional cropping alternated with a minimum of 3 yr of pasture would maintain soil properties within acceptable limits and meet the goals of sustainable agriculture under conditions similar to this experiment.